Add a demonstration of NetCDF integer packing to lfric_apps rose-stem

### Are there any linked Issues or Pull Requests?

_No response_

### Brief description

Integer packing appears to be a performant and effective way of reducing data volume used by the LFRic model.

A full description can be found in MetOffice/tcd-IO-performance/issues/55

**We would like to add the testing of integer packing to the LFRic apps rose-stem**

### Further details of the issue.

NetCDF allows the packing of floating point fields into integer fields. By NetCDF convention these integer fields have two double precision parameters:

- `scale_factor` (_S_)
- `offset` ($\Delta$)

We pack the floating point data $D_\text{float}$ by

$$
D_\text{packed} = \frac{D_\text{float}}{\Delta} - S
$$

and casting to either a 8 bit or 16 bit integer. To reconstruct the data, we do the reverse process.

XIOS allows us to access the packing functionality, by specifying `prec=2` (16-bit integer), and the variables `add_offset` and `scale_factor` within field references. For example

```

```
may become
```

```

Start by reading MetOffice/tcd-IO-performance issue 55 and the open pull request #542, then inspect the lfric_apps rose-stem configuration where XIOS field references are defined. Verify how the supplied add_offset, scale_factor, prec="2", and conversion_by_netcdf settings are exercised. Done means the rose-stem testing demonstrates NetCDF integer packing for the relevant floating-point fields.

Written by the indexing model from the issue text.